Annealing furnaces are used in order to expose workpieces after the actual production or manufacturing in a controlled manner to a heating that improves the material properties.
In particular, stainless steel tubes manufactured by cold forming, i.e., for example, by cold pilgering or cold drawing, are annealed after the forming in an annealing furnace in order to increase the ductility of the material. To generate the temperatures needed for annealing steel tubes, it is sufficient for the annealing furnace to comprise a furnace muffle base body that is manufactured from metal or from another inexpensive available material that can be brought into nearly any shape.
However, it has been found that the base bodies of furnace muffles themselves undergo a considerable deformation, due to the heating of the volume delimited by them. This deformation is further increased since the furnaces are not operated continuously but are switched off temporarily to save energy and since they cool off during that period. Owing to these cooling and heating cycles, clear deformations of the furnace muffle occur.
The consequence of such a deformation of the furnace muffle or of their base bodies is that the muffle is subjected to increased wear and has to be replaced soon by a new muffle. In addition, in furnaces where the muffle itself is heated from the outside, i.e., the base body of the muffle is used as a radiation source for heating the volume enclosed by it, a deformation of the furnace muffle leads to the heating of the volume of the furnace becoming inhomogeneous, and the tempering or the annealing of the material becoming inefficient.